This invention relates to inertial instrument sensors. In particular, the present invention is a ring laser gyroscope dither spring having a mounting mechanism and method for accurately securing piezoelectric transducers to the dither spring reeds.
A ring laser gyroscope (RLG) is commonly used to measure the angular rotation of a vehicle, such as an aircraft. Such a gyroscope has two counter-rotating laser light beams which move within a closed loop optical path or "ring" with the aid of successive reflections from multiple mirrors. The closed path is defined by an optical cavity which is interior to a gyroscope frame or "block". In one type of RLG, the block includes planar top and bottom surfaces that are bordered by six planar sides that form a hexagon shaped perimeter. Three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the comers of the optical path which is triangular in shape.
Operationally, upon rotation of the RLG about its input axis (which is perpendicular to and at the center of the planar top and bottom surfaces of the block), the effective path length of each counter-rotating laser light beam changes and a frequency differential is produced between the beams that is nominally proportional to angular rate. This differential is then measured by signal processing electronics to determine the angular rotation of the vehicle.
Because of backscatter radiation, which is created as the laser light beams are reflected at the mirror surfaces, and other factors, the frequency difference between the counter-rotating laser light beams disappears when the angular velocity of the RLG about its input axis has a value that is below a particular threshold. This phenomenon is called "lock-in", and the range of angular rotation over which lock-in occurs is the "deadband" of the RLG. This phenomenon is undesirable because, at low rotation rates, lock-in produces an indication that no rotation is occurring when in fact, there is low rate angular rotation. Therefore, any inability to accurately measure low angular rotation rates reduces the effectiveness of the RLG in vehicle navigation systems.
There are several known approaches to eliminating the lock-in phenomenon. One such approach involves using a drive motor for mechanically oscillating the RLG about its input axis so that the RLG is constantly sweeping through the deadband and is never locked therein. This mechanical oscillation of the RLG is called dithering.
Dithering is accomplished by mounting gyroscope block on a flexure device known as a "dither spring". The dither spring is generally composed of a central member or hub (which is centered on The RLG input axis) having a plurality of flexible radial members or reeds extending between the hub and a toroidal rim. Each reed has a pair of piezoelectric transducers (PZT's) mounted on opposite sides thereof via an adhesive. The combination of the dither spring and PZT's defines the drive motor for mechanically oscillating the RLG about its input axis.
Voltages are applied to the PZT's such that one PZT on each reed increases in length while the other PZT decreases in length. The effect of these length changes in the PZT's is transmitted to the reeds through the mounting of the PZT's thereon. Increasing the length of one side of each reed while shortening the other side causes the reeds to flex or bend so that each reed experiences a small rotation about the RLG input axis. The voltage is oscillatory so that the reeds are constantly vibrating in phase and the gyroscope block mounted to the toroidal rim rotates about the input axis. The amplitude of the dithering is generally carefully controlled and monitored to minimize the effects of lock-in. Since the dither oscillation angular velocity and displacement can be constantly monitored, they can be excluded from the output signal of the RLG.
To attain proper dither frequency so as to eliminate the effects of lock-in, and to minimize material stress of the dither spring due to the high frequency flexing of the reeds, the PZT's must be precisely mounted with respect to one another on the reeds of the dither spring. The adhesive bond between the PZT's and the dither spring reeds must exhibit high strength and uniformity across the PZT/reed interface in order to withstand the high mechanical stresses generated between the PZT's and dither spring during dithering. Presently, positioning of the PZT's on the dither spring reeds is accomplished by a person using a combination of sighting by eye and assembly tooling to achieve precise placement and bonding of the PZT's. However, since PZT positioning accuracy and bonding uniformity is largely dependent upon the skill of the person doing the assembling, precise placement and consistent uniform bonding of the PZT's on the dither spring reeds is not always achieved. If the accuracy of the PZT placement is not within acceptable parameters, and/or the adhesive bond across the PZT/reed interface not uniform, RLG performance is compromised and the dither spring and RLG must be rebuilt or scrapped. This increases the manufacturing cost of the dither spring as well as the RLG.
There is a need for an improved mounting mechanism and method for accurately securing and uniformly bonding the PZT's to the reeds of a dither spring of a RLG. In particular, there is a need for a PZT mounting mechanism and method that enables a person assembling the PZT's to the dither spring to consistently, precisely position and uniformly bond the PZT's on the dither spring reeds. In addition, the mounting mechanism and method should improve the consistency of PZT placement and the uniformity of the PZT/reed adhesive bond interface so as to reduce the number of dither springs needing to be rebuilt or scrapped. Lastly, the PZT mounting mechanism and method should be relatively easy and inexpensive to practice and should greatly facilitate automation of assembly.